Automated and semi-automated techniques have been developed to test integrated circuits (ICs). Causes of faults or errors encountered during testing are, however, notoriously difficult to trace.
For example, an IC may receive an analog system reference voltage, and may include one or more voltage dividers to generate one or more additional analog voltages from the system reference voltage. Other circuitry within the IC may utilize one or more of the analog voltages as reference voltages, bandgap reference voltages, and/or bias voltages. An analog voltage may encounter a voltage or IR drop due to distance from a driver or buffer circuit, loading effects of associated circuitry, and/or fabrication variations that may affect characteristics of the die itself. An IR drop may impact operation of the IC in one or more of a variety of ways, including slowed response times and loss of functionality, which may be detected as an error or fault during testing of the IC.
To trace or debug the source of the error or fault, on-die analog signals may be measured with an off-die measurement device, such as an oscilloscope or volt meter. Conventionally, multiple on-die signals are multiplexed to one or more off-die accessible pads through analog multiplexers, often through relatively long and circuitous signal routes, which may itself introduce IR drops, capacitive loads, and/or other effects, and which may mask true states of the on-die signals.
In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.